Sensitive sound fault meter and detector utilizing a double side band frequency current-generator

ABSTRACT

An electrical leakage detector indicating hazard level, for use in hospitals or the like, using a constant current generator having at least two side band frequencies on either side of the line frequency with a high impedance return to the ground from the constant current generator, together with a discriminator means to distinguish between the power line frequency and the two side band frequencies and for indicating departure of current from predetermined limits.

United States Patent [191 Peterson [54] SENSITIVE SOUND FAULT METER ANDDETECTOR UTILIZING -A DOUBLE SIDE BAND FREQUENCY CURRENT-GENERATOR [75]Inventor: Wesley G. Peterson, Mount Prospect, Ill.

[73] Assignee: 801:! Basic Industries, Inc., Milwaukee, Wis.

22 Filed: Apri127,1 971 21 Appl.No.: 137,814

52 U.S.Cl ..324/5l,3l7/l8D,340/255 51 lnt.Cl. ..G0lr3l/02 [58]FieldofSearch ..324/s1,s2,54;3r7/1s;

[56] References Cited UNITED STATES PATENTS 2,315,383 3/1943 Andrews..324/52 1 Jan. 9, 1973 2,315,450 3/1943 Nyquist ..324/52 2,627,5402/1953 Rich 324/51 2,832,916 4/1958 Kennedy.... ..324/5l X 2,999,2319/1961 Kusters et a ..324/5l X 3,066,284 11/1962 McKinley et a1...324/51 X Primary Examiner-Gerard R. Strecker Attorney-Smythe & Moore[57] ABSTRACT 6 Claims, 4 Drawing Figures IZWWB DOUBLE SIDE BAND CURRENTGENERATOR SIDE BAND VOLTAGE Z --DETECTOR 2 l4 l7 l5 PATENTEBJAI 9 i975SHEET 1 OF 2 SIDE BAND VOLTAGE DETECTOR F G .l l3

DOUBLE SIDE BAND CURRENT GENERATOR COM. 6RD,

2. SHIELD GUARD OUTPUT l VENTOR WESLKY 6 PETERSQ/V BY COM GRD.

ATTORNEYj SENSITIVE SOUND FAULT METER AND DETECTOR UTILIZING A DOUBLESIDE BAND FREQUENCY CURRENT-GENERATOR This invention relates to hazardlevel indicators and particularly to a low leakage current or groundfault indicator or detector for use in hospital operating rooms or thelike.

Previously, various systems have been used for determining leakagebetween an ungroundecl system and the ground. Some of these systems usean arrangement for sequentially connecting each of the lines to groundand indicating or detecting the amount of current flowing through thecircuit when it reaches high values. In areas, such as hospitaloperating rooms, where there is a danger of explosion from gases orshock, electrical codes provide for the use of isolated or ungrounded.

detector includes a constant current generator which has constantcurrent output at two side band frequencies, one of which is on one sideof the power line frequency and the other on the other side. The outputof the constant current generator is connected to the lines beingmonitored. A high impedance return connection is provided from theconstant current regulator to the ground so that the impedance is suchthat the detected current will be extremely small. A discriminator meansis used to distinguish between the side band frequencies so as to givean indication if the leakage level reaches an undesirable high value.

One of the purposes is to continuously monitor the absolute value of theline frequency leakage path from the circuit to ground which mightpresent a clinical hazard to patients.

Other objects, advantages and features of the invention will becomeapparent from the following description and drawings, which are merelyexemplary.

In the drawings:

FIG. 1 is a schematic block diagram illustrating the principles of theinvention;

FIG. 2 is a block diagram showing one form of carrying out theinvention;

FIG. 3 is a fragmentary schematic diagram illustrating one form of anattenuator that may be used; and

FIG. 4 is a fragmentary schematic diagram showing a guard system thatcan be used.

Referring to the schematic illustration of FIG. 1, conindicated by theboxes 14 and 15. The voltage from node 16 to ground is measured by asuitable instrument 17. In case of a leakage occuring in either line 12or 13, there will be an indication in 17 of the change in impedance.

It would be desirable to make the leakage impedance measurement at 60Hz, but an active 60 Hz line itself introduces 60 Hz, which is difficultto separate from the desired signal. For this reason, the 60 Hz signalcontributed by the line is carefully filtered out by an active filter.Additionally, all line frequency harmonics, especially the third at Hz,are also filtered. In order to make the measurement, a pseudo 60 Hzsignal is created which effectively mimics 60 Hz for measurementpurposes but is not discriminated against by the filter chain. This newsignal is composed of two equal amplitude signals placed in frequencyabove and below 60 Hz, i.e., 55 Hz and 65 Hz, and produced in the outputof a balanced modulator as side bands by a 5 Hz oscillator modulating a60 Hz carrier. A low-pass filter follows the modulator to removespurious side bands about the odd harmonics of the carrier.

Referring now to FIG. 2, power lines L and L are the lines beingmonitored. As mentioned, the invention will be described as applied to a60 Hz supply with the side band frequencies at fi Hz. It is to beunderstood that other frequencies and side bands can be used. The groundconnection for the system is indicated at G. The various parts shown anddescribed in the drawings comprise known electrical circuits except asmay be described specifically hereafter. Oscillator 21, which is a 5 Hzoscillator, is used to create the side bands in conjunction with theline frequency introduced by lines L and L through the balancedmodulator 20. In the particular example shown, the oscillator provides adouble side band of 5 Hz on either side of the 60 Hz signal linefrequency which is seen in an isolating transformer 22. In effect, the60 Hz signal is a carrier with a modulation signal having a 5 Hz signalon either side so as to provide a 55 Hz and 65 Hz signal but notincluding the 60 Hz signal. Such can be termed a double side bandsuppressed carrier. Low-pass filter 24 eliminates the higher order ofside band signals harmonically related to the carrier frequency. Thevoltage at point 25 from low-pass filter 24 contains only the 55 Hz and65 Hz frequencies.

The voltage at point 25 is fed through impedance 26 to the summingjunction 27 of operational amplifier 28. The summing junction 27 isassumed to be at virtual ground potential so that all of the voltage 25is across impedance 26 to cause a current to flow into the summingjunction. Virtually all of the current at the summing junction 27 flowsthrough line 29 to the secondary of transformer 30. From the secondary,it is fed through impedance 31, consisting of resistor and capacitor, tothe common node or center tap 32 of transformer 22. Eventually, it isfed to the 60 Hz isolated lines. Operational amplifier 28 provides thatthe potential of the summing junction 27 is maintained at virtualground. This is obtained by closing the loop around the operationalamplifier through the transformer 30. Because the summing junction 27 isheld stable by the loop, constant current is available at the output ofthe constant current generator at 31B irrespective of load.

Thus, it can beseen that the output e, of the side band generator drivesoperational amplifier 28. The output signal of amplifier 28 at thesecondary of 30 is a current of magnitude i te /Z Because of the currentfeedback loop around the amplifier, i is nearly independent of theexternal loop.

As an example, the source impedance for the current generator can be inthe order of 200 megohms. Thus, the current is impressed upon the twoisolated lines L L from point 32. The voltage which appears betweenpoint 32 and ground is measured by a very high impedance volt meter,which also has an impedance of the order of 200 megohms, the volt meterconsisting of high impedance attenuator 33, a series of filtersincluding 60 Hz two-pole low-pass filter 34, 180 Hz notch filter 35, 60Hz notch filter 36, 60 Hz band pass filter 37, 60 Hz notch filter 38,which is the same as notch filter 36, and a 60 Hz band filter 39, whichis the same as band pass filter 37. The output of band pass filter 39 isthen fed to a detector which converts the ac. voltage signal to d.c.,the detector being an absolute value detector 40. Filters 34, 35, 36,37, 38 and 39 serve as a discriminator to distinguish between the sideband frequencies and the line frequency. The output of the absolutevalue detector is fed to logarithm converter 41 and then to meteramplifier 42 and to a standard d.c. milliameter 43. The logarithmconverter may consist of log converter 41A and integrator 413. Themilliamater 43 can be a two decade log scale which can'measurc,

for example, from microamps of leakage to 3,000 microamps.

Summarizing, the current introduced to the ac. lines in common nodes bymeans of the primary center tap node 32 of transformer 22 must find areturn path through whatever leakage impedance exists from either lineto ground. The very high impedance attenuator is placed from the samepoint node 32 on the transformer to ground or in parallel with theleakage impedance to be measured. The output of the attenuator is asignal containing line frequency and double side band signal voltagewherein the side band voltage is proportional to the leakage impedance.After filtering to remove the unwanted signals, the desired signal isfull wave detected in an absolute value detector. The detector andrelated parts convert the side band signal to a d.c. level with-an a.c.ripple equal to the difference frequency of 10 Hz plus side bandcomponents. The log converter takes the logarithm of this signal,integrates out the higher order side bands and subtracts the d.c.component and '10 Hz from a constant voltage k. Such is equivalent tostating that if the smoothed d.c. voltage E is proportional to theleakage impedance Z,

then k log E-k log Z= log I log Z= log K/Z where K is a new constant andthe output of the log converter V-log K/Z. If the new constant Krepresents the nominal line voltage of, for example, 120 VAC, 'Vis avoltage which is proportional to the logarithm of the leakage currentbecause this is defined to be equal to K/Z. In this manner, it isconvenient to display two decades of leakage current with constantprecision on a d.c. meter with a log scale. In the present example, thescale is calibrated 0.03 to 3.0 milliamperes.

Additionally, an alarm circuit can be provided. If a predetermined highleakage current is indicated, one type of indication will be provided,and if the leakage current is below a predetermined value, another typeof indication will be given. Such can be accomplished by a voltagecomparator 44 driving a solid state relay 45 which can operate a signalarrangement 46. The reference for the voltage comparator is set by thetrip level pot 47.

FIG. 3 shows the circuitry of one form of active attenuator that can beused. Common node 31B is connected through the impedance 31A, whichincludes a resistor and capacitor (FIG. 2), to the secondary 59 of a lto 20 turns ratio transformer 60. The transformer has bifilar woundprimaries 61, 62. One side of primary 61 is connected to ground, and theopposite side is connected through line 63 to point 64 (FIGS. 2, 3) soas to appear as the attenuator output, which in turn is connected tolow-pass filter 34. Primary 62 also is connected to ground, and theother side is driven by the output of amplifier 65 which is similar tooperational amplifier 28 (FIG. 2); The configuration of the attenuatormay be said to be identical to the configuration of the currentgenerator described previously, which includes impedance 26, amplifier28, transformer 30 and feedback 29, with the exception that the inputpoint of the current generator 26 is referred to ground rather thandriven by some voltage. This provides the same type of active sourceimpedance as would be found from the current generator, e.g., 200megohms, and also provides the 20 to l attenuation factor of the inputsignal by the attenuator to output 64. The various impedances 65A, 66,67 and 68 are used for stabilization purposes of the attenuator.Impedance 70 is analogous to impedance 26 of FIG. 2.

A guard system can be used in conjunction with the 20 to 1 activeattenuator to cancel the effect of interwinding capacitance and windingto core capacitance, of each of the transformers in the circuit, e.g.,transformers 22, 30 and 60. These connections and elements are shownschematically as 22A and 22B, 30A and 30B, and 60A and 603. Since eachtransformer in the circuit otherwise would introduce leakage of its ownwhich would be analogous to external leakage and would beindistinguishable, each transformer can thus I 71 of transformer 70which is almost precisely the same as thevoltage being measured-so thatit is a source that can be used to drive the guard or electrostaticshields of the transformers as mentioned.

- The potential leakage hazard measurement may be made therefore in amanner which itself introduces a maximum measurement leakage currentwhich may be,

for instance, less than one-sixth of the least potential current to bemeasured. f I

The guard output also could be used to guard the system power wires aswell by connecting the voltage to the shields of the power wires.

It is to be understood that variations may be made in details ofconstruction and arrangement of circuits without departing from thespirit of the invention except as set forth in the appended claims.

I claim:

1. In a leakage detector for a power system having two isolatedconductors to be monitored, said power system having a predeterminedfrequency, the combination including a constant current generator havingan output of at least two side band frequencies, one above and one belowsaid predetermined frequency, means connecting one side of the output ofsaid current generator to earth ground, means for feeding the other sideof the output of said current generator through a transformer means tosaid two conductors, discriminator means, means connecting saidtransformer means to said discriminator means, said discriminator meansdistinguishing between said side band frequencies and the frequency ofthe line being monitored, and means connected to said discriminatormeans to indicate the impedance of the power system to earth ground.

2. In a leakage detector as claimed in claim 1 and including means toindicate when the impedance to earth ground is more 05 less than apredetermined value.

3. In a leakage detector as claimed in claim 1 and wherein the constantcurrent generator includes an operational amplifier in a closed loop.

4. In a leakage detector as claimed in claim 3 wherein the constantcurrent generator is driven by double side band signal means.

5. In a leakage detector as claimed in claim 1 wherein the discriminatormeans includes filter means to distinguish between the side bandfrequencies and said predetermined frequency.

6. In a leakage detector as claimed in claim 1 and including means forproviding guard voltage means to drive guard shields between circuitmeans connected to said isolated conductors and other components of saidleakage detector.

1. In a leakage detector for a power system having two isolatedconductors to be monitored, said power system having a predeterminedfrequency, the combination including a constant current generator havingan output of at least two side band frequencies, one above and one belowsaid predetermined frequency, means connecting one side of the output ofsaid current generator to earth ground, means for feeding the other sideof the output of said current generator through a transformer means tosaid two conductors, discriminator means, means connecting saidtransformer means to said discriminator means, said discriminator meansdistinguishing between said side band frequencies and the frequency ofthe line being monitored, and means connected to said discriminatormeans to indicate the impedance of the power systEm to earth ground. 2.In a leakage detector as claimed in claim 1 and including means toindicate when the impedance to earth ground is more os less than apredetermined value.
 3. In a leakage detector as claimed in claim 1 andwherein the constant current generator includes an operational amplifierin a closed loop.
 4. In a leakage detector as claimed in claim 3 whereinthe constant current generator is driven by double side band signalmeans.
 5. In a leakage detector as claimed in claim 1 wherein thediscriminator means includes filter means to distinguish between theside band frequencies and said predetermined frequency.
 6. In a leakagedetector as claimed in claim 1 and including means for providing guardvoltage means to drive guard shields between circuit means connected tosaid isolated conductors and other components of said leakage detector.